Method for processing glass filament

ABSTRACT

A method of processing glass filament comprises: providing a length of glass filament from which a portion is to be separated from the remainder of the filament; directing energy onto the filament in order to cause a decrease in a width of the filament at a desired location for separation of the portion; and causing relative longitudinal movement between the portion and the remainder of the filament to separate the portion from the remainder of the filament at the desired location.

BACKGROUND OF THE INVENTION

The present invention relates to methods for processing glass filament,in particular for separating portions such as canes from a glassfilament. The glass filament may be filament for forming optical fibre.

Optical fibres are typically manufactured by assembling a plurality ofglass components into a preform assembly, which has a cross-sectionalstructure corresponding to the desired structure for the fibre, and amuch greater diameter than the desired fibre diameter. The preform is“drawn down” in a fibre drawing tower by heating the preform to softenthe glass and pulling a continuous filament from it, maintaining thesame cross-section but at a reduced diameter. The smaller diameter maybe the intended fibre diameter, but commonly the diameter isintermediate between the preform size and the intended fibre size. Inthe latter case, the filament is separated into shorter portions,conveniently carried out as the filament is drawn from the preform. Theshorter portions are termed canes, and in turn will be drawn down into acontinuous filament of optical fibre.

Originally, optical fibres had a solid structure, so that the preformcomprised a solid glass rod corresponding to the desired cross-sectionof the core and inner cladding for the finished fibre, inserted into ahollow tube destined to form the fibre's outer cladding. More recentfibre designs employ longitudinal holes or lumina that extend the lengthof the fibre and define the structure of either or both of the core andthe inner cladding. A preform for such fibres can be made by stackinghollow tubes and capillaries, and optionally solid rods, into thedesired cross-sectional pattern.

One technique for separating a filament into individual canes is knownas cleaving, which is a mechanical technique also suitable for preparingthe end faces of optical fibres. A notch, scratch, crack or score isformed in the outer glass surface of the filament (using a saw, abrasivewheel, diamond blade, ceramic blade or steel blade, for example), and atensile stress is induced in excess of the tensile strength of the glassin a region adjacent to the tip of the notch. This causes the notch topropagate through the glass, thereby separating a cane from theremainder of the filament at the position of the notch. Cleaving is ableto produce end surfaces which are flat, smooth, and at a controlledangle, such as perpendicular, to the longitudinal axis of the filament,and with edges that are generally clean and free from chips or blemishes(expect, generally, at the position of the notch). Nevertheless,cleaving and other mechanical techniques can be problematic for theseparation of canes from a filament. If chips or blemishes do arise atthe cleaved surfaces, it may not be possible to successfully draw thecane into a fibre, because cracks may propagate from the flaws. Also,debris may be deposited on and around the cane end. This is a particularrisk for fibre structures comprising one or more lumina, since debrismay enter the lumina. Debris can cause defects in fibre subsequentlydrawn from the cane, which may manifest as increased optical loss ormechanical weakness of the fibre. All these issues can require thescrappage of significant lengths of fibre and cane, thus reducing yieldand increasing cost.

An additionally issue is potential perturbation of the glass filament ifcane separation is carried out in a draw tower as the filament isformed. Perturbation may locally change the diameter of the cane fromthe desired diameter.

Accordingly, alternative methods of processing glass filament toseparate canes from the filament are of interest.

SUMMARY OF THE INVENTION

Aspects and embodiments are set out in the appended claims.

According to a first aspect of certain embodiments described herein,there is provided a method of processing glass filament, comprising:providing a length of glass filament from which a portion is to beseparated from the remainder of the filament; directing energy onto thefilament in order to cause a decrease in a width of the filament at adesired location for separation of the portion; and moving the portionaway from the remainder of the filament to obtain the portion separatedfrom the remainder of the filament.

According to a second aspect of certain embodiments described herein,there is provided a cane suitable for drawing into an optical fibre andwhich has been obtained by separation from a glass filament using amethod according the first aspect.

According to a third aspect of certain embodiments described herein,there is provided a device for drawing glass filament from a preform,comprising: a pair of rotatable belts each having a movable surfacerotatable about an axis of rotation, the belts arrangeable intopositions in which the movable surfaces are facing with a separation toreceive and grip the glass filament, movement of the surfaces acting topull the filament away from the preform along a direction of alongitudinal axis of the filament; in which the axes of rotation of thebelts are locatable at opposite and substantially equal non-orthogonalangles to the pull direction, in order to impart a rotation to thefilament about its longitudinal axis.

These and further aspects of certain embodiments are set out in theappended independent and dependent claims. It will be appreciated thatfeatures of the dependent claims may be combined with each other andfeatures of the independent claims in combinations other than thoseexplicitly set out in the claims. Furthermore, the approach describedherein is not restricted to specific embodiments such as set out below,but includes and contemplates any appropriate combinations of featurespresented herein. For example, methods and apparatus may be provided inaccordance with approaches described herein which include any one ormore of the various features described below as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings in which:

FIG. 1 shows a flow chart of a method for processing a glass filamentaccording to an example of the present disclosure;

FIG. 2 shows a perspective side view of a glass filament to whichmethods of the present disclosure may be applied;

FIGS. 3A, 3B and 3C show transverse cross-sectional views of examplefilaments such as that of FIG. 2 ;

FIG. 4 shows a schematic side view of apparatus for processing afilament according an example of the present disclosure;

FIG. 5A shows a schematic representation of an example energy source andbeam directing system which may be included in the apparatus of FIG. 4 ;

FIG. 5B shows a cross-sectional view of an elongate focussed spot of anenergy beam generated by the beam directing system of FIG. 5A;

FIGS. 6A, 6B and 6C show schematic representations of further examplesof an energy source and beam directing system which may be included inthe apparatus of FIG. 4 ;

FIG. 7 shows a schematic side view of alternative apparatus forprocessing a filament according to an example, at an intermediate pointof a processing method performed on a glass filament;

FIG. 8 shows a schematic side view of the apparatus of FIG. 7 or FIG. 4, at a further intermediate point in a processing method that includesthe use of crimping jaws;

FIG. 9 shows a schematic side view of the apparatus of FIG. 7 , at anend point of the processing method showing a cane portion separated fromthe filament;

FIG. 10 shows a schematic side view of further alternative apparatus forprocessing a filament according to a further example method, at anintermediate point in the method;

FIG. 11 shows a schematic side view of the apparatus of FIG. 10 , at anend point of the processing method showing a cane portion separated fromthe filament;

FIG. 12 shows a schematic side view of an example vapour handling systemsuitable for inclusion in the apparatus of FIGS. 10 and 11 ;

FIG. 13 shows a schematic side view of a modified version of theapparatus of FIG. 10 ;

FIGS. 14A and 14B show orthogonal side views of a cane puller by whichglass filament can be pulled or drawn from a glass preform; and

FIGS. 15A and 15B show orthogonal side views of an example cane pulleraccording to an aspect of the present disclosure.

DETAILED DESCRIPTION

Aspects and features of certain examples and embodiments arediscussed/described herein. Some aspects and features of certainexamples and embodiments may be implemented conventionally and these arenot discussed/described in detail in the interests of brevity. It willthus be appreciated that aspects and features of apparatus and methodsdiscussed herein which are not described in detail may be implemented inaccordance with any conventional techniques for implementing suchaspects and features.

The present disclosure presents methods for processing glass filament byseparating a cane for forming into optical fibre from the remainder of alength of the filament, using the application of energy in place ofmechanical techniques known to produce debris and flawed end surfacesthat compromise the quality of optical fibre drawn from the cane. Theuse of energy for cane separation can produce clean end surfacessubstantially free from debris and flaws, and in some cases closed toprevent the ingress of contaminants. Also, mechanical techniques such ascleaving can be poorly suited for cutting through larger diameters offilament, whereas the energy-based approaches can be adapted to filamentdiameter by appropriate selection of the energy characteristics.

FIG. 1 shows a flow chart setting out steps in an example method forglass filament processing as described herein. In a first step S1, themethod comprises providing a glass filament. This is a length offilament, formed from a glass material such as fused silica, that has across-sectional structure appropriate to a desired structure for anoptical fibre but having a larger diameter than that intended for thefibre. The filament will typically have been fabricated by drawing itfrom a glass preform or preform assembly, which, again, has anappropriate structure but on a still larger scale, and which is producedby stacking and otherwise arranging glass tubes, capillaries and rods toform the desired structure. Drawing a filament from a preform can becarried out in a draw tower, as will be understood by the skilledperson. In order to make the final optical fibre, the filament will bedivided or separated into individual shorter lengths or portions, whichcan be termed “canes”, each of which can be drawn into optical fibre.The method of FIG. 1 shows steps to achieve separation of a cane fromthe rest of the filament, and may be carried out within a draw tower onfilament freshly drawn from a preform, so the processing removessuccessive cane portions from the filament as it is drawn.Alternatively, the filament may be drawn from the preform as acontinuous length, and be processed into separated portions as aseparate stage, outside the draw tower.

In a second step S2, energy is applied to the filament. This may be donein a variety of ways, as described further below. The energy is directedonto the filament at a desired location which divides the filament intoa part or portion which is to be separated off to form a cane, and theremaining part or portion of the filament (from which more canes can beseparated by repeating the method). In a third step S3, the appliedenergy is used to introduce a reduction in the width or diameter of thefilament at the desired location. As will be understood, the supply ofenergy to the glass material causes material changes to the glass, whichcan be handled and directed appropriately to form a narrowed part of thefilament. Techniques for achieving this are described further below.

In a fourth step S4, the cane portion of the filament and the remainderportion of the filament, in other words, the portions of the filament oneither side of the reduced width desired location, are moved apart.Specifically, the cane portion is moved away from the remainder portion,and in this way the separated cane portion is obtained, separated fromthe remainder portion at the desired location. The movement of the caneportion can be achieved in various ways.

In one example, the application of energy in step S3 is carried out toreduce the diameter of the filament to a non-zero value, in other words,the filament is thinner at the desired location, but the cane portion isstill unitary with the remainder portion. The cane portion is thenseparated from the remainder portion by introducing a relativelongitudinal movement between the two portions, where the longitudinaldirection is the direction along the length of the filament (along itslongitudinal axis). One or both of the portions may be moved, dependingon where the processing is being performed. In a draw tower, forexample, both portions may be advancing as the filament is drawn fromthe preform, and the relative movement is introduced by increasing theadvancement of the cane portion at the remote end of the filament. Thereduced width and/or the change in the fabric of the glass at thedesired location introduces a weakness into the filament; the glass hasa reduced structural integrity at this point. Accordingly, moving thetwo portions apart from one another causes the glass to break, snap,sheer, pull apart or otherwise divide, so that the cane portion becomesseparated from the remainder portion of the filament. A similar divisionmay be achieved by relative movement between the portions along adifferent direction, such as a sideways or bending movement of the caneportion.

In another example, the application of energy in step S3 is carried outto reduce the diameter of the filament to zero, so that the cane portionis non-unitary with the remainder portion but still immediately adjacentthereto. Typically this may be achieved by applying more energy at thedesired location, such as energy delivered at a higher power or for alonger time. The energy application alone is hence sufficient to causecomplete separation or severance of the cane portion from the remainderportion. The movement in step S4 is therefore a removal of the caneportion from its position adjacent to the remainder portion, so that thecane portion is isolated from the remainder portion and placed in astate suitable for storage or further processing. The movement may ormay not be relative longitudinal movement between the two portions, asin the previous example.

FIG. 2 shows a schematic perspective side view of a length of glassfilament 10 to which methods of the present disclosure may be applied.The filament 10 has a length L (extending above and below the depictedportion of filament), a longitudinal axis A and a width or diameter W.The filament 10 is nominally divided into a cane portion 12 and aremaining or remainder portion 14 at a desired location D, passingthrough the filament 10 typically orthogonally to the longitudinal axisA. The remainder portion 14 may or may not extend from a preform orpreform assembly (not shown) from which the filament 10 is being drawn.Methods of the present disclosure relate to the separation of the caneportion 12 from the remainder portion 14 at the desired location D, andmay be carried out either during or after drawing of the filament 10from its preform.

The filament 10 has a cross-sectional structure formed from the glassmaterial that corresponds, on a larger scale, to the desired structureof the finished optical fibre. The methods herein are applicable to anyfibre structure, and are not limited in this respect.

FIG. 3 shows some examples of possible cross-sectional structures of thefilament 10. FIG. 3A shows a structure for forming an all-solid opticalfibre, the filament therefore comprising a solid glass rod 16 in whichthe core and cladding(s) of the fibre structure are defined by differentvalues of refractive index. FIG. 3B shows a structure for a simplehollow core fibre, the filament comprising a glass tube 18 with a hollowcentre 20 that will form the core of the fibre. FIG. 3C shows astructure for a more complex hollow core optical fibre, in which thefilament comprises a hollow glass tube 18 that will form an outercladding of the fibre, or a portion thereof, and a plurality of nestedpairs of hollow glass capillaries 22 arranged in a ring around theinterior of the tube 18, which will form an inner cladding of the fibre.A hollow space 20 remains inside the ring of capillaries 22, which willform the hollow core of the fibre. This particular design of hollow corefibre can be described as a nested antiresonant nodeless fibre (NANF).

The example structures of FIGS. 3B and 3C therefore comprise one or morelumina, being holes or hollow spaces that extend longitudinally alongthe filament, and also in the completed fibre formed from a cane cutfrom the filament. Any other lumen-based structure may also be processedby currently-proposed methods. These include other antiresonant hollowcore fibre structures which may include more or fewer nested orun-nested capillaries, spaced or unspaced, in the ring for the innercore, hollow core photonic bandgap (or photonic crystal) fibrestructures which comprise an inner cladding defined by a periodic arrayof lumina or capillaries, and kagome fibre structures which also utiliseperiodic arrangements of lumina. The disclosure is in no way limited inthis regard, and may be applied to glass structures for any opticalfibre design.

FIG. 4 shows a schematic representation of apparatus arranged to carryout an example method of separating a cane from a filament according tothe present disclosure. A glass filament 10, shown from the side, whichmay have any cross-sectional structure as described above with regard tothe FIGS. 2 and 3 , is positioned for performing the separation. Asdepicted, the filament 10 is arranged vertically, such as it would be ifbeing drawn in a downwards direction from a preform (not shown) in adraw tower. If not actively being drawn, the filament may be suspendedin a draw tower after drawing from the preform, or during a pause indrawing. Alternatively, the filament may be processed after drawing,remotely from the draw tower. In this case, the filament may bevertically oriented as depicted, but may equally be processed in ahorizontal position, or an intermediate position, as convenient.

A location on the filament 10, a specified distance from the free(lower) end of the filament (not shown) that corresponds to the requiredlength of a cane, is designated as a desired location D, being a desiredlongitudinal location or position at which the filament 10 is to beseparated into a cane portion and a remaining filament portion. Thefilament is secured for processing by a first clamp arrangement 24comprising one or more clamps or clamping devices that hold the filament10 on one side of (in this example, above) the desired location D. Thefirst clamp arrangement 24 therefore holds the portion of the filament10 that will become the remaining portion 14 after separation. A secondclamp arrangement 26 comprising one or more clamps or clamping devicesholds the filament 10 on the other side of (in this example, below) thedesired location. The second clamp arrangement 26 therefore holds theportion of the filament 10 that will become the cane portion 12 afterseparation. At an initial stage, before the application of energy to thefilament 10, the clamps are separated by a distance d1.

Energy is applied to the filament from an energy source. The purpose ofthe application of energy is to deliver energy into the glass materialto increase its temperature and cause a change in state that results ina change of shape of the filament. The state change can be softening orablation, as will be described further below. Accordingly, any form ofenergy that is able to produce this effect can be used. In the exampleof FIG. 4 , the energy source is a laser 28 which emits a beam of laserlight 30. In this simple example, the beam 30 is focussed with one ormore lenses 32 in order to form a focal spot 34 which is directed ontothe outer surface of the filament 10 in line with the desired locationD.

The lenses 32 can be considered to be a beam directing system, which canbe configured in a variety of ways depending on the nature of the energyand the energy source, the size of the filament and where the separationmethod is carried out. In the case of a beam of laser light, the beammay be focussed as in FIG. 4 , to form a focal spot at the filamentsurface that is substantially circular in shape, or substantiallylinear, or some intermediate or other shape. Alternatively, the beamdirecting system may substantially collimate the energy towards thefilament, where the collimated beam may have a circular, linear,intermediate or other cross-sectional shape. Usefully, the energy can bedelivered so as to be distributed around the circumference of thefilament 10, rather than to a single point as in depicted in FIG. 4 ,although this arrangement may be practical if sufficient energy can bedelivered in a localised manner. Accordingly, the energy can be appliedto one or more regions circumferentially disposed around the filament atthe desired location; the regions may be discontinuous spots, or mayhave the form of a continuous or substantially continuous ring, whichmay be formed from simultaneous delivery around the circumference, ormade up of contiguous or overlapping regions to which the energy isapplied.

FIG. 5A shows a simplified schematic representation of an example beamdirecting system 32 for delivering a laser beam 30 from a laser 28. Thebeam 30 is emitted from the laser source 28 with a circular beamcross-section, and passes through the beam directing system 32 thatcomprises a negative cylindrical lens 36 followed by a positivecylindrical lens 38, with their axes parallel. These lenses 36, 38 actto shape the beam into an elongated elliptical cross-section, which ispassed through a spherical lens 40 to a focus 34 directed onto thefilament.

FIG. 5A shows the resulting elliptical focal spot 34. Usefully, the longaxis of an elliptical spot can be arranged to be orthogonal to thelongitudinal axis of the filament, so that the spot can extendsignificantly around the circumference of the filament at the desiredlocation, without reaching far along the length of the filament aboveand below the desired location. This helps to deliver the energyefficiently to the desired location.

If a laser is used as the energy source, its optical wavelength, outputpower and operating regime (such as continuous wave or pulsed output)can be selected with reference to the filament properties, such asdiameter and glass composition, and the configuration of the beamdelivery system, in order to maximise the efficiency of the delivery ofan appropriate amount of energy to the filament. As an example, acontinuous wave carbon dioxide laser may be used, operating at 10.6 μmor 9.3 μm. Alternatively, the laser output may be in the form ofultrashort pulses, of picosecond or femtosecond duration. Other usefulwavelengths are in the green region of the spectrum, such as at oraround 532 nm.

The energy source need not be a laser, however. In other examples, theenergy source may a flame source, delivering energy in the form of aflame, such as a hydrogen-oxygen flame. A further alternative is the useof plasma as the energy, delivered for example from an energy source inthe form of a plasma torch.

FIG. 6A shows a schematic representation, as a view from above/belowalong the longitudinal axis of a filament, of a further example beamdirecting system, configured to deliver optical energy in the form of alaser beam simultaneously to three regions circumferentially disposedaround the filament. The initial output beam 30 from a laser 28 isincident on a first beam splitter BSa. This divides the light into twoparts, one of which, 30 a is reflected by the beam splitter BSa anddirected by a first pair of mirrors Ma1 and Ma2 to a first region on thefilament 10, via a first lens La which focuses the beam 30 a to a focalspot 34 a at the surface of the filament. A second part of the originalbeam 30 is transmitted by the first beam splitter BSa to a second beamsplitter BSb, which divides the light into two parts. One part 30 b isreflected by a second beam splitter BSb to a second pair of mirrors Mb1and Mb2 which direct the beam part 30 b to a second region on thefilament 10, via a second lens Lb which focuses the beam 30 b to a focalspot 34 b on the surface of the filament 10. Finally, a remainder partof the light 30 c is transmitted by the second beam splitter BSb towardsa third region on the filament 10, via a third lens Lc which focuses thebeam 30 c to a focal spot 34 c on the surface of the filament 10. Thethree regions 34 a, 34 b, 34 c are evenly spaced apart around thecircumference of the filament 10, at the desired location D. Hence, agreater proportion of the circumference is exposed to the laser beamenergy than would be possible with a single beam. The exposure can beenhanced if the lenses La, Lb, Lc are configured to shape the beams intoelongate focal spots as in the example of FIG. 5B; multiple elongatespots can be arranged to wrap around most or all of the circumferenceand therefore expose the filament all around the desired location. Ifthe specification of the beam splitters is chosen appropriately, eachbeam part can comprise the same amount of laser energy in order todeliver a more even distribution of energy to the filament, but this isnot essential.

In other examples the laser beam may be divided into more or fewerparts, by providing more or fewer beam splitters and lenses. Each partcan be directed onto a different circumferentially-spaced region on thefilament; if the spacing is even and each beam part has roughly the sameenergy an even distribution of energy is delivered around the desiredlocation. The lenses and plane mirrors of the FIG. 6A example may bereplaced with non-plane mirrors, for example paraboloidal mirrors. Also,separate lasers may be used to deliver each beam part directly, so thatbeam splitters to divide the beam are not needed.

Depending on factors including the focal spot size, the amount of energyin the beam parts, and the size and structure of the filament,delivering the energy statically to one or more regions around thecircumference of the filament may be suitable for achieving the requiredprovision of energy to the glass. In other cases, it may be moresuitable to ensure that more regions, or the full circumference, receiveenergy. To achieve this, an entire beam delivery system such as thatshown in FIG. 6A can be configured to be rotatable around an axiscoincident with the longitudinal axis of the filament, to as to tracethe focal spot or spots of the beam or beam parts fully or partly aroundthe filament. A continuous circumferential region of energy exposuremight be provided in this way, or an circumferential arrangement ofspaced-apart regions if the energy is not delivered continuously.Alternatively, the various groups of mirrors and lenses may be moved inorder to direct their corresponding beam parts to different regions atthe desired location, for example in a scanning arrangement.

FIG. 6B shows a schematic representation, as a side view, of anotherexample beam directing system, configured to deliver optical energy inthe form of a laser beam over a substantially continuous ring around thefilament. A plane mirror 82 is arranged to direct the output beam 30from a laser 28 substantially coaxially with the filament 10 to form areflected beam 88. The output beam 30 is configured to have a diametergreater than the width of the filament 10 in order that a substantialportion may pass the filament 10 to reach the plane mirror 82. The planemirror 82 is provided with a hole 84 or other means of allowing thefilament 10 to pass through it. An optional baffle 86 may be included toblock the part of the output beam 30 that would otherwise be incident onthe filament 10, allowing the rest of the output beam to propagate pastthe baffle 86 to reach the plane mirror 82. The reflected beam 86, whichhas the form of a cylinder substantially surrounding the filament 10,propagates along the length direction of the filament to be incident ona paraboloidal mirror 90. As with the plane mirror 82, the paraboloidalmirror 90 is provided with a hole 92 or other means of allowing thefilament 10 to pass through it. The paraboloidal mirror 90 is configuredto focus the reflected beam 88 to a ring around the filament 10,focussed at or near the filament's outer surface at the desired locationD. The ring focus may not be continuous around the filament, for exampleowing to an area in the reflected beam 88 being shadowed by the filament10 or the baffle 86, full circumferential exposure of the filament tothe laser energy at the desired location D may be achieved by relativerotational movement of the beam directing system and the filament.

If a beam directing system such as the examples of FIG. 6B and 6C isemployed while the filament is being drawn from a preform assembly in adraw tower, it is useful to have the paraboloidal mirror 90 mounted on acarriage 94 arranged for movement parallel to the draw direction and thefilament's longitudinal axis

FIG. 6C shows a schematic representation, as a side view, of a yetfurther example beam directing system, in this case configured todeliver optical energy in the form of a laser beam swept around asubstantially continuous ring around the filament, at the desiredlocation D. The system comprises a plane mirror 84 and a facingparaboloidal mirror 90 arranged annularly around the filament 10 as inthe FIG. 6B example. Prior to the plane mirror 84, the system isdifferently configured, however. The output beam 30 from a laser 28 isdirected along a propagation direction substantially parallel to butoffset from the longitudinal axis of the filament 10, to a planescanning mirror 96. The beam waist of the output beam 30 may bepositioned coincident with the surface of the scanning mirror 96. Thescanning mirror 96 is mounted on a shaft 98 driven by a motor 100 forrotational movement centred on the point of incidence of the output beam30 on the scanning mirror 96. However, the scanning mirror 96 is mountedsuch that there is an angle between the axis of the shaft 98 (axis ofrotation) and the normal to the surface of the mirror 96. Thus, as theshaft 98 and scanning mirror 96 rotate, the propagation direction givento the beam 102 reflected from the scanning mirror changes according tothe rotational position of the shaft 98. This changing propagationdirection is indicated in FIG. 6C by the reflected beam 102 a at a firstposition, and the reflected beam 102 b in phantom at a second position.The reflected beam 102 therefore travels over the surface of a cone, asthe scanning mirror 96 rotates. Alternative ways to scan the reflectedbeam in a similar manner include the use of a prism and a prism pair(Risley prisms). The reflected beam 102 is incident on an off-axisparaboloidal mirror 104, which may if necessary have a hole 106 or othermeans of allowing the output beam 30 from the laser 28 to pass throughit. The paraboloidal mirror 104 is arranged such that the doublyreflected beam 108 leaving its surface has a propagation directionindependent of the position of the shaft 98, in order to alwayspropagate along a same parallel direction to the plane mirror 82, whichdirects the beam 88 to the focussing paraboloidal mirror 90 to focus thebeam to the desired location D, as before. The changing propagationdirection of the reflected beam 102 leaving the scanning mirror 96causes the focus of the beam at the filament surface to sweep around thecircumference of the filament. In this way, the energy can be exposedonto the filament along a continuous ring around the filament, withoutthe need to rotate either the beam directing system or the filament.

As noted the energy need not be optical energy in the form of a laserbeam. Similar delivery arrangements that allow simultaneous exposure ofmultiple regions around the filament can be provided for other energytypes. For example, if the energy is derived from a flame, several flamesources can positioned around the filament, optionally in a rotatablearrangement. If the energy is a plasma delivered as a plasma jet from aplasma torch, several plasma torches can be positioned around thefilament, again optionally in a rotatable arrangement.

FIG. 7 shows a schematic side view of example apparatus similar to thatshown in FIG. 4 , but after energy has begun to be or has been appliedto the filament 10. In this example, the energy source is a flame 40,delivered to one (as shown) or multiple regions around the filament 10at the desired location D. In this example, the amount of energy whichis delivered is selected with reference to the properties andcharacteristics of the filament 10 in order to cause a softening orpartial melting of the glass material of the filament at the desiredlocation. The softening enables deformation of the filament 10 such thatits width W is reduced at the desired location, to form a neck, waist or“necked down” part or region having a smaller, non-zero, width W′. Thedeformation can be enabled by any of various means. If the filament hasa cross-sectional structure including one or more lumina, the softeningcan cause distortion of the internal structure and collapse of thelumen, so that the outer wall of the filament also collapses inwards andreduces the width.

FIG. 8 shows a schematic side view of an example apparatus for enablingthe reduction in width. After the glass has softened (and convenientlybut not necessarily after the energy delivery is complete), crimpingjaws 42 or a similar device can be applied to the filament at thedesired location and operated to apply an inward pressure in the planeof the desired location (a pinching or compressing action) in order toreshape the glass into a narrower form and create the neck. This isapplicable for both solid and hollow or partly hollow filaments.

Otherwise, some relative movement can be made between the cane portion12 and the remaining portion 14 to produce or enhance the narrowing ofthe width. The movement might be longitudinal, along the axis of thefilament, so that the softened glass is stretched over a large lengthand therefore will adopt a smaller width, or might be rotation toproduce a twisting action that will deform and compress the softenedglass.

Once the narrowed neck portion has been created, and while the glass isstill in a softened state (hence energy may continue to be applied tomaintain the increased temperature), the cane portion 12 is separatedfrom the remainder portion 14 of the filament 10.

FIG. 9 shows the example apparatus of FIG. 7 after separation has beeneffected. To produce the separation, the cane portion 12 and theremainder portion 14 are moved apart from one another in thelongitudinal direction. The movement is relative, in that one or bothportions may be moved. Within the present example, the movement can beeffected by increasing the distance between the first clamp arrangement24 and the second clamp arrangement 26, from the spacing dl of FIGS. 4and 7 to a larger spacing d2. The softened glass material will stretchand neck down to increasingly narrow widths until the material of thetwo portions 12, 14 pulls apart and the portions separate, as shown inFIG. 9 .

Alternatively, the amount of energy applied to soften the glass,optionally aided by crimping jaws or the like, may be enough of itselfto produce the separation, by reducing the filament down a necked regionwith a zero width. The movement between the portions can then beperformed to move the already-separated cane portion away from theremainder portion and to a different location.

If the softened glass approach is used, the glass of the outer surfaceof the filament moves inwardly and eventually closes over the end ofeach of the portions 12, 14 leaving a teardrop or ogive shape at theportion ends. In general, this is beneficial in that the end surfaceflaws that can be introduced by mechanical cane separating techniquesare eliminated. In the specific case of a filament with one or morelumina in its interior structure, the closed shaping of the glass at theend of the cane portion and the end of the remainder portion acts toclose or seal the lumina. This prevents the ingress of contaminants intothe lumina, both during the separation process and during later stagessuch as storage and further processing. The quality of optical fibresubsequently drawn from canes produced in this way is thereforepotentially improved over that from mechanically-separated canes.

The amount by which the spacing or separation d1 has to be increased tod2 in order to cause separation will depend on the diameter of thefilament and the size of the reduced width W′; a thicker filament mayrequire a greater relative longitudinal movement to cause the width toneck down to zero, at which point separation occurs.

Once separated from the filament, the cane portion or cane can be movedto a different location for storage or further processing, such as by amanipulator or robotic arm, in line with known cane and fibre drawingprocedures. The second clamp arrangement 26 may be used for thispurpose.

As an alternative, one or other of the first clamp arrangement and thesecond clamp arrangement may be omitted, and the filament secured forprocessing by a single clamp arrangement only. In such a configuration,if the filament is secured in a vertical orientation as in the Figures,and as it would be in a draw tower, gravity can be used to effect themovement between the cane portion and the remainder portion of thefilament. Once the glass at the desired location has been softened, thecane portion will drop under its own weight by the action of gravity,perhaps assisted by the weight of the second clamp arrangement if thisis the one clamp arrangement used, until the neck portion thins to zerowidth and the cane becomes separated. Rapid application of sufficientenergy may create a zero-width neck portion before the cane portionstarts to drop, so that the gravitational motion moves thealready-separated portions apart.

FIG. 10 shows a simplified schematic view of apparatus for performing amethod of cane separation according to a further example. As with themethod described with respect to FIG. 4 , initially a filament 10 issecured vertically, such as inside a draw tower, by a first clampingarrangement 24 at an upper position and by a second clamping arrangement26 at a lower position, on either side of a desired location D whichdivides the filament 10 into a remainder filament portion 14 and a caneportion 12 which is to be separated from the remainder filament portion14. The first and second clamping arrangements are separated by aspacing dl, as before.

Again as previously, the apparatus includes an energy source in the formof a laser 28 which emits a beam of laser light 30 which is focussed anddirected onto the filament surface at the desired location D by a lensarrangement 32 which forms a focussed spot 34. As before, the laserenergy source can be configured with an appropriate beam directingsystem to apply one, two or more focussed or collimated spots of lightaround the circumference of the filament 10, with the option of rotatingaround the filament in order to deliver light to any or allcircumferential regions. Similarly, the energy source may alternativelycomprise one or more flame sources or plasma sources.

This example differs from previous examples in that the energy isprimarily delivered in such a way as to ablate the glass material of thefilament at the desired location, rather than to soften the glassmaterial. For softening, the state change of the glass material ismelting, moving from a solid glass to a softened, near-liquid glass dueto an appropriate temperature increase caused by the deposited energy.In contrast, ablation causes a state change of the glass material fromsolid to gas or plasma, depending on the density of the deliveredenergy. At lower densities or fluxes the solid glass is converted to agas by evaporation or sublimation, whereas a higher density can convertthe solid material directly to a plasma. Accordingly, ablation causesthe physical removal of material from the filament. The removal can belimited to the outer or surface layer or layers of the filament, and inthis way, the width of the filament is reduced at the desired location,in line with, but via a different mechanism from, the width reduction bynecking down achieved in the glass softening example. Alternatively, theablation may remove enough material to effectively cut through thefilament, reducing the width to zero at that point.

The skilled person will be able to select appropriate operatingparameters for the energy source in order to deliver the energy to thefilament in a way that will produce ablation rather than softening. Ingeneral, a higher energy density will be required to achieve ablationthan to produce softening.

The ablation is carried out by removing material so as to create a cut,groove or slot 44 in the filament, by directing energy to the desiredlocation. The slot 44 is preferably circumferential in orientation, inthat it is aligned around the circumference of the filament,substantially perpendicular to the longitudinal axis of the filament. Byrelative rotational movement of the applied spot or spots of the energybeams, such as the laser beam 30 of the FIG. 10 example, around thefilament (as previously described), one or more slots 44 can be createdat the desired position. The slot 44 may be cut as a continuous slotaround the filament, for example by sweeping a single beam right aroundthe filament, or as a series of overlapping or adjacent slots each cutby a different one of multiple beams spaced apart around the filament.

A reduction in filament width to a non-zero width caused by the presenceof the slot or slots 44 weakens the filament at the desired location.Accordingly, the filament can be broken at the desired location bycausing a relative longitudinal movement between the cane portion 12 andthe remainder portion 14 so as to move the two portions apart.

FIG. 11 shows the apparatus of FIG. 10 with the cane 12 separated fromthe remaining portion 14 of the filament, following a relativelongitudinal movement of the two portions corresponding to an increasein the spacing of the clamping arrangement spacing from d1 to d2 Thismay be carried out after the application of energy to cause theablation, so the energy source (laser) 28 is shown in a non-operationalcondition, with no output beam. The strain caused by the longitudinalmovement causes the filament to preferentially snap at the weakenedpoint of the desired location, allowing the cane 12 to separate from theremaining portion of the filament 10. The slot (s) 44 may be cut to anydepth, with a slot depth which is a greater proportion of the filamentwidth increasing the weakness at the desired location and hencefacilitating separation of the cane 12. A convenient depth for hollowcore filament is approximately the wall thickness of the outer glasstube. Alternatively, the slot or slots may be sufficiently deep to forma reduced width of zero and completely sever the cane 12 from thefilament.

As discussed with regard to FIG. 9 , one clamping arrangement only maybe used to secure the filament in a vertical orientation, and the forceof gravity used to effect the relative longitudinal movement to separatethe cane.

The use of energy, delivered as laser light or a flame or plasma, tocause ablation allows the slot to be cut without any generation of soliddebris such as chips of the glass filament material which may be createdwhen mechanical cleaving techniques are used which rely on saws,abrasive wheels, or diamond, ceramic or steel blades.

In contrast with the glass softening embodiment, which allows the end ofthe cane to be closed thereby sealing any lumen, the ablation approachallows cane separation to be achieved with minimal distortion of theinternal structure of the filament, so that lumina can be maintainedopen and the end parts of a cane are able to be drawn into opticalfibre. Maintaining some or all of the lumina open is relevant for fibredrawing methods in which one or more internal pressures are controlled.This reduces waste and allows fibre output to be maximised. Theparameters of the applied energy can be selected with reference to theinternal filament structure in order to minimise distortion of theinterior walls and closure of the lumen. In other words, the energyapplication may be formulated in order to maximise the ablation effectand minimise melting or softening of the glass.

As mentioned, ablation by the application of a laser beam or other formof energy is a process that typically produces vapour or plasma. Anyvapour may subsequently condense. In the present case, the condensationmay be on the outer surface of the filament, on the end surface asexposed by the cut slot, and if there are lumina, on inner surfaces ofthe filament. This condensation is effectively a contamination, in thatmaterial (albeit glass material from the filament itself) is depositedwhere it should not be present. This can affect the drawing of theseparated cane into a fibre, and modify the fibre's optical propertiessuch as giving an increased optical propagation loss. Accordingly, it isproposed that the by-products of ablation be removed as they aregenerated, in order to reduce condensation and maintain the filamentquality.

Returning to FIGS. 10 and 11 , the apparatus additionally comprises avapour handling system 50, shown in simplified form, which comprises anair handling unit 52 and ducting 54 connected to the air handling unitat one end. The other, remote, end (or ends) 56 of the ducting 54 isplaced adjacent to the filament 10, proximate the desired location D andthe region of the filament 10 to which the energy spot 34 is applied.

FIG. 12 shows a schematic simplified view of an example air handlingunit, in more detail. The vapour handling unit 50 includes a first airhandling unit 52 a with an associated first duct or ducts 54 a. Thefirst air handling unit 52 a delivers clean, dry air (or an alternativegas such as nitrogen) through the first duct 54 a, which is an emissionduct to one or more nozzles 58 a, 58 b at the remote end 56 of the firstduct 54 a. In the case of two or more nozzles, these may be arrangedaround the circumference of the filament 10, proximate the desiredlocation D and slightly spaced apart from the filament surface.Alternatively, the nozzles 58 a, 58 b may be in the form of a slot orslots arranged circumferentially about the axis of the filament 10. Thenozzles 58 a, 58 b emit the air delivered from the first air handlingunit 52 a as air flow 60 a, 60 b from each respective nozzle; the airflow 60 a, 60 b is arranged to impinge on the filament 10 in such amanner as to entrain vapour and/or particles generated by the impact ofthe energy (not shown) which applied to the filament 10 at the desiredlocation D. The air flows 60 a, 60 b carry the entrained vapour andparticles (which we may commonly term debris or by-products of theablation) so as to move them away from filament 10. For example, theflowing air may hit the filament surface and bounce off it in an outwarddirection, carrying the debris with it. In this way, the debris is takenout of the vicinity of the filament 10, and the risk of the debrisdepositing onto the outer and/or inner surfaces of the filament 10 isreduced. To enhance the effectiveness of this removal, the air handlingcapability of the vapour handling unit 50 may be supplemented by asecond air handling unit 52 b equipped with an extraction duct 54 c. thesecond air handling unit 52 b is an air intake unit rather than an airemission unit. The remote end 56 of the extraction duct 54 c has anozzle 58 c positioned appropriately to collect the flowing air 60 a, 60b that has entrained the vapour and particles. More than one extractionduct 54 c may be provided, or the extraction duct 54 c may have morethan one end nozzle 58 c.

Returning to FIG. 10 , note that the ablating energy, such as the laserbeam 30, is delivered onto the filament surface along a direction whichis substantially orthogonal to the longitudinal axis of the filament(horizontal in the depicted orientation of a vertical filament). In theevent that the filament 10 is has an internal cross-sectional structurecomprising a lumen, such as the example of FIG. 3 c , it will beapparent that the outer tube 18 will be cut through before the walls ofthe inner capillaries 22. If the energy is in a beam 30 brought to afocal spot 34 at the desired location D, the energy beam 30 will divergeafter the focal spot 34 before passing through the filament 10. Thediverged beam will be incident on the inner structure of the filamentincluding the opposite inner surface of the outer tube 18. For ahorizontally directed beam, the divergent energy will spread both aboveand below the desired location, and hence be incident on both the caneportion 12 and the remaining filament portion 14. This may generatevapour which will be inaccessible to the vapour handling system 50, andwhich can hence subsequently condense inside the both the cane portion12 and the remaining filament portion 14. Hence, both portions of thefilament 10 may be internally contaminated.

FIG. 13 shows a simplified schematic side view of apparatus modified toaddress this circumstance. The beam directing system (comprising lens 32in the depicted simple arrangement, but which may be considerably morecomplex and configured to handle multiple beams or have multiple beamdirections as described with regard to FIGS. 6A, 6B or 6C) is arrangedsuch that the energy beam or beams 30 are directed in a downwardlysloping direction (relevant to the depicted vertical orientation of thefilament 10). This allows the focal spot 34 to be incident on theexterior of the filament 10 at the desired location D to effectseparation of the cane portion 12 at the correct portion, while thediverging beam after the focal spot 34 is incident on the interiorstructure of the filament 10 over a region or regions 35 (shaded in FIG.13 ) which are substantially or entirely within the cane portion 12,that is, below the desired location D. In this way, the condensation ofany vapour will be confined substantially within the cane portion 12.

While the examples thus far have been described largely within thecontext of separating a cane from a larger length of glass filament,either while the filament is being drawn from a preform in a draw tower,or after drawing, the separation methods are not limited in this regard.In particular, any of the examples and modifications thereof may be usedto cut an existing cane to a different length by removing a portion atone or both ends of the cane. This might be to achieve a particularlength of cane for example. In the case that the softened glassseparation method has been used to acquire a cane from a drawn filament,so that the cane has a closed or necked-down end, the ablationseparation method may be used to remove the end parts, for example. Insuch a case, the angled beam application described with respect to FIG.13 may be used to limit internal vapour condensation to the end partwhich is being removed, thereby avoiding contamination of lumina in thefinished cane. Accordingly, references in the preceding description to“filament” can apply to filament that has been or is being drawn from apreform and which require cutting into canes or other shorter lengths,and also to an already separated cane from which a part or parts are tobe separated. References to “cane portion” can refer to a cane beingseparated from a filament, or to any part being separated from a cane,such as a necked or closed end part. References to “remaining portion ofthe filament” (and similar terms) can apply to filament remaining afterseparation of a cane, or to the bulk of a cane remaining after theseparation of one or both end parts. The methods may also be used forcutting finished optical fibre to desired lengths or trimming off endparts.

The use of a single clamping arrangement (or similar securing or holdingdevice) and the force of gravity to cause the separation by relativelongitudinal motion, as already described, may be useful and convenientin the context of removing portions from existing canes.

When a method according to the examples described herein is employedwhile the filament is being drawn from a preform assembly in a drawtower, apparatus for enabling the method can include a system or devicefor pulling the filament downwardly from the preform to achieve acontinuous draw, as is well understood.

FIGS. 14A and 14B show simplified schematic views of an example devicefor this purpose, commonly termed a “cane puller”. The device 70comprises a pair of rotatable belts 72, each of which has the form of acontinuous loop that passes around a pair of spaced apart drive wheels74 that hold the belt in a taught configuration. Rotation of the twodrive wheels in the pair in the same direction pulls the belt around andaround in the same direction. The axis X about which this rotationoccurs lies midway between the axes of the two drive wheels. The twobelts 72 are arranged in an opposing configuration with their axes ofrotation X parallel so that the outwardly facing surfaces 76 of thebelts 72, which each provide a moving surface when the belt 72 rotates,face one another to provide opposing parallel moving surfaces 76. Theopposing moving surfaces 76 are spaced apart by a width W (convenientlythe relative position of the belts 72 can be adjusted to move the beltsinto and out of this alignment) corresponding to the width of thefilament 10. Accordingly, the filament 10 can be gripped between the twoopposing surfaces 76 of the belts 72, oriented with its longitudinalaxis A orthogonal to the rotation axes X of the belts 72, and parallelto the direction of movement of the moving surfaces 76. If the drivewheels 74 of one belt 72 are driven in the opposite direction to thedrive wheels 74 of the other belt 72, the two moving surfaces 76 willmove in the same direction, as indicated by the arrows. Accordingly, thefilament 10 will be continuously fed through the space between themoving surfaces 76, in that same direction (downwardly in the depictedorientation). This will continuously pull fresh filament 10 from apreform (not shown), thereby providing a continuous supply of newfilament 10 from which canes can be separated.

The drive wheels 74 are under control of a drive mechanism (not shown)that may include a computerised controller programmed to provideautomated control of the pulling of the filament 10.

FIG. 14A shows a side view of the cane puller 70, in a plane transverseto the rotation axes X of the belts 72. FIG. 14B shows the orthogonalside view of the cane puller 70 (without any filament), in a planeparallel to the rotation axes X of the belts 72. From this it can beappreciated that the two belts 72 have the same alignment (since one isconcealed behind this other from this viewing angle), in that their axesof rotation X are parallel to one another and orthogonal to the filamentaxis A, providing moving surfaces that move in a direction also parallelto the filament axis.

FIGS. 15A and 15B show simplified schematic side views of an examplecane puller according to an aspect of the present disclosure. The canepuller 80 comprises a pair of opposing rotatable belts 72 a, 72 bconfigured and operable in the manner described with regard to FIGS. 14Aand 14B to provide a pair of opposing movable surfaces 76 spaced by adistance W that act to grip and continuously feed a filament 10 in adownward direction, by the belts 72 a, 72 b being driven in oppositedirections. However, in this example, the axes of rotation Xa, Xb of thebelts 72 a, 72 b are not parallel to one another. Instead they lie inparallel planes which are also parallel to the axis A of the filament,but each is positioned at a substantially equal but oppositenon-orthogonal angle to the axis A of the filament (and hence also tothe direction of travel or feed of the filament through the canepuller). This can be appreciated from FIG. 15B, which shows that eachbelt 72 a, 72 b is tilted away from the vertical direction (filamentaxis A) within the plane of the moving surfaces 76, but in oppositedirections. Hence, there is an angle Y between the two axes of rotationXa, Xb. The axis of rotation Xa of one belt 72 a lies at an angle Y′above the horizontal direction (orthogonal to the filament axis A) whilethe axis of rotation Xb of the other belt 72 b lies at the same angle Y′below the horizontal direction, where 2Y′=Y. Similarly, the two axes ofrotation lie at equal and opposite non-orthogonal angles, 90°-Y′, to thefilament axis A, which is also the longitudinal direction along whichthe filament is pulled or drawn from the preform. This may also bedescribed as the directions of travel of the moving surfaces 76 of thetwo belts 72 a, 72 b being separated by the angle Y, each lying at anangle Y′ on either side of the filament axis A.

The effect of this angled positioning of the two rotating belts is toimpart a twisting motion to the filament 10 as it is fed downwardsthrough the cane puller 80, indicated by the spiral arrow R in FIG. 15A.The filament 10 therefore rotates about its longitudinal axis A as it ispulled through the cane puller 80. This motion can be used to providethe relative rotational movement between the applied energy beam(s) andthe filament in order to deliver to energy circumferentially around thefilament at the desired position, as described above. Accordingly, abeam directing system 32 such as those shown in FIGS. 4, 5A, 6A, 6B and6C may remain stationary while the filament rotates about its axis A.Alternatively, the two motions may be combined, with the beam directingsystem 32 rotating about the filament in a first direction, and thefilament rotating within the beam directing system, in a second oppositedirection.

The cane puller 80 may be configured such that the angles of the beltsare fixed, so that a constant twisting motion is provided duringfilament draw. Alternatively, the belts may be movable (for exampleunder control of a same controller as operates the rotation of therotating belts) so that their relative angles may be adjusted. This canallow the filament rotation to be turned on and off (by switching thebelt rotation axis angles between non-orthogonal and orthogonal to thefilament axis), or turned between rotation in one direction and rotationin the opposite direction (by reversing the non-orthogonal belt rotationaxis angles), or changed in speed (by making the non-orthogonalbelt-rotation axis angles smaller or larger).

Other methods for producing rotation of the filament within the energybeam directing system may be alternatively be used.

The filament processing methods proposed herein are applicable generallyto filaments comprised of glass materials. The filament may be made frommaterials known for the fabrication of existing designs of solid andlumen-containing optical fibres, in particular glass materials such assilica. For internally structured filaments, the various tubes andcapillaries may be made from the same material or from differentmaterials. Types of glass include “silicate glasses” or “silica-basedglasses”, based on the chemical compound silica (silicon dioxide, orquartz), of which there are many examples. Other glasses suitable foroptical applications and from which the filament may usefully be madeinclude, but are not limited to, chalcogenide, tellurite glasses,fluoride glasses, and doped silica glasses. The glass materials mayinclude one or more dopants for the purpose of tailoring the opticalproperties, such as modifying absorption/transmission or enablingoptical pumping.

The methods are also generally applicable to a wide range of filamentswidths or diameters, such as from typical optical fibre diameters, forexample about 100 μm, up to typically cane diameters of 20 mm or more.

The various embodiments described herein are presented only to assist inunderstanding and teaching the claimed features. These embodiments areprovided as a representative sample of embodiments only, and are notexhaustive and/or exclusive. It is to be understood that advantages,embodiments, examples, functions, features, structures, and/or otheraspects described herein are not to be considered limitations on thescope of the invention as defined by the claims or limitations onequivalents to the claims, and that other embodiments may be utilisedand modifications may be made without departing from the scope of theclaimed invention. Various embodiments of the invention may suitablycomprise, consist of, or consist essentially of, appropriatecombinations of the disclosed elements, components, features, parts,steps, means, etc., other than those specifically described herein. Inaddition, this disclosure may include other inventions not presentlyclaimed, but which may be claimed in the future.

1. A method of processing glass filament, comprising: providing a lengthof glass filament from which a portion is to be separated from theremainder of the filament; directing energy onto the filament in orderto cause a decrease in a width of the filament at a desired location forseparation of the portion; and moving the portion away from theremainder of the filament to obtain the portion separated from theremainder of the filament.
 2. A method according to claim 1, in whichthe separated portion forms a cane able to be drawn into optical fibre.3. A method according to claim 1, in which the energy is in the form ofone or more laser beams or flames or plasma beams applied to thefilament.
 4. A method according to claim 3, in which the energy isdirected to at least one focus substantially at the surface of thefilament at the desired location.
 5. A method according to claim 1, inwhich directing energy onto the filament comprises directing energy ontotwo or more regions circumferentially disposed around the filament atthe desired location.
 6. A method according to claim 1, furthercomprising causing relative rotational movement between the filament andthe energy while directing the energy onto the filament, in order todirect energy onto a region or regions circumferentially disposed aroundthe filament at the desired location.
 7. A method according to claim 1,in which directing energy onto the filament causes the width of thefilament to decrease to zero at the desired location, thereby separatingthe portion from the remainder of the filament.
 8. A method according toclaim 1, in which directing energy onto the filament reduces the widthof the filament at the desired location to greater than zero, and movingthe portion away from the remainder of the filament comprises causingrelative longitudinal movement between the portion and the remainder ofthe filament to separate the portion from the remainder of the filamentat the desired location.
 9. A method according to claim 8, furthercomprising, during the directing energy, holding the remainder of thefilament with a first clamp arrangement at a first position, and holdingthe portion with a second clamp arrangement at a second positionlongitudinally spaced from the first position, and causing the relativelongitudinal movement by increasing the spacing between the first clamparrangement and the second clamp arrangement.
 10. A method according toclaim 8, further comprising, during the directing energy, holding one ofthe remainder of the filament and the portion with a clamp arrangement,and causing the relative longitudinal movement by allowing the portionto move under gravity.
 11. A method according to claim 1, comprisingdirecting energy onto the filament that is sufficient to cause softeningand deformation of the glass of the filament into a necked region at thedesired location.
 12. A method according to claim 11, further comprisingapplying crimping jaws to the softened glass to aid deformation into thenecked region.
 13. A method according to claim 11, in which the filamenthas an internal structure comprising one or more longitudinal lumina,and deformation of the glass into the necked region comprises collapseof the one or more lumina.
 14. A method according to claim 13, in whichthe deformation and optionally also the moving of the portion away fromthe remainder of the filament cause a width of the necked region todecrease to zero, thereby closing the lumina at the separated end of theportion.
 15. A method according to claim 1, comprising directing energyono the filament that is sufficient to cause ablation of the glass ofthe filament at the desired location.
 16. A method according to claim15, in which the glass is ablated to form a circumferential slot aroundthe filament at the desired location.
 17. A method according to claim15, further comprising removing vapour and/or debris generated by theablation by providing a flow of air to carry vapour and/or debris awayfrom the filament.
 18. A method according to claim 15, in which thefilament has an internal structure comprising one or more longitudinallumina, and directing energy onto the filament comprises directingenergy at a non-orthogonal angle to the longitudinal axis of thefilament and towards the portion so as to reduce condensation of vapourgenerated by the ablation inside the lumina in the remainder of thefilament.
 19. A method according to claim 1, in which the length ofglass filament has been drawn from a glass preform and the method iscarried out in a draw tower while the filament is being drawn from theglass preform.
 20. A method according to claim 19, in which the filamentis drawn from the glass preform using a cane puller comprising a pair ofopposing rotating belts arranged to grip opposite sides of the filamentand pull the filament in a longitudinal direction away from the preform.21. A method according to claim 20, in which the pair of opposingrotating belts are arranged such that the axis of rotation of each beltis orthogonal to the longitudinal direction.
 22. A method according toclaim 20, in which the pair of opposing rotating belts are arranged suchthat the axes of rotation of the belts are at opposite and substantiallyequal non-orthogonal angles to the longitudinal direction, in order tocause rotation of the drawn filament about its longitudinal axis.
 23. Acane suitable for drawing into an optical fibre and which has beenobtained by separation from a glass filament using a method according toclaim
 1. 24. A cane according to claim 23, having an internal structurecomprising one or more longitudinal lumina and having one or more closedends.
 25. A device for drawing glass filament from a preform,comprising: a pair of rotatable belts each having a movable surfacerotatable about an axis of rotation, the belts arrangeable intopositions in which the movable surfaces are facing with a separation toreceive and grip the glass filament, movement of the surfaces acting topull the filament away from the preform along a direction of alongitudinal axis of the filament, in which the axes of rotation of thebelts are locatable at opposite and substantially equal non-orthogonalangles to the pull direction, in order to impart a rotation to thefilament about its longitudinal axis.
 26. A device according to claim25, in which the axes of rotation of the belts are additionallylocatable parallel to one another and substantially orthogonal to thepull direction, in order to pull the filament without imparting arotation.
 27. A device according to claim 25, in which the opposite andsubstantially equal non-orthogonal angles can be altered, in order tomodify a speed of the rotation, including being reversed in order toreverse a direction of the rotation.